This invention relates to a detector and alarm circuit for use with electric apparatus which is connected to power lines, and where the body of the apparatus is insulated from ground. More particularly, this invention relates to a detector and alarm circuit for electrically powered mobile apparatus such as trolley buses and the like for determining incipient leakage resistance between the body of the apparatus and the power lines such that when the body is insulated from ground and one of the power lines is substantially at ground potential, a voltage occurs on the body of the apparatus which might be hazardous to any human who might come into contact with it.
One such leakage resistance detector and alarm circuit is shown in our earlier U.S. Pat. No. 3,987,425. According to this patent, the circuit provides a warning alarm after experiencing a voltage above a particular level for certain duration and once so sensed, a second delay circuit is activated and if the voltage remains for a second duration, an alarm is activated.
In trolley cars, for example, the overhead lines are such that one is normally at ground potential and the other is at about 600 volts. Leakage will occur between the electrical connections of the lines to the trolley car and the apparatus body will be raised from its normal ground potential. This potential remains on the body as the body is isolated from ground. A certain minimum voltage is normal, however, as this voltage continues to climb, the danger to passengers leaving or entering the car increases as they may provide an electrical connection between the ground and the vehicle body. The danger to the passengers dramatically increases as the voltage of the apparatus body exceeds a predetermined minimum, normally about 30 volts In the case of a complete failure where the apparatus body would be at several hundred volts, the passenger is in extreme danger.
This prior art leakage resistance detector and alarm circuit does not take into account the magnitude of the voltage sensed, and the time duration between the first occurance of the sensed voltage and the time of initiation of a warning is not variable in accoardance with the magnitude of the voltage.